Barrier coatings may decrease permeation of gases and vapors through polymeric materials by several orders of magnitude. Such coatings are used to produce materials that may replace glass in food and pharmaceutical packaging, and in electronic applications. They are also used as protective coatings against attack by aggressive liquids, gases or vapors. One particularly important and demanding application is related to light emitting or photovoltaic devices based on organic semiconductors.
An organic light emitting diode (OLED) device is an emissive display in which a transparent substrate is coated with a transparent conducting material, for example, indium-tin oxide (ITO) which forms a hole-injecting electrode as the lowest layer of a light emitting diode. The remaining layers of the diode, commencing with the layer adjacent the ITO layer, comprise a hole-transporting layer (HTL), an electron-transporting layer (ETL) and an electron-injecting electrode.
The hole-transporting layer is essentially a p-type semi-conductor and the electron-transporting layer is essentially an n-type semi-conductor. These are organic layers and in particular are conjugated organics or conjugated polymers; the latter are poor conductors without dopants but are doped to conduct holes (p-type) or electrons (n-type).
The electron-injecting electrode is typically a metal such as calcium, lithium or magnesium.
When a voltage is applied to the diode, electrons flow towards the hole-transporting layer and holes flow towards the electron-transporting layer. This produces electron-hole recombinations which release energy as light. Collectively the hole-transporting layer (HTL) and the electron-transporting layer (ELT) form the electroluminescent layer (EL) of the diode.
Such OLEDs provide a new generation of active organic displays of high efficiency, and most important of low cost. In those displays, high-quality images are created by a matrix of the light emitting diodes encapsulated in transparent materials.
The diodes are patterned to form a pixel matrix, where a single-pixel junction or EL emits light of a given color. All organic displays, designed so far, contain oxygen- and moisture-sensitive components, namely organic semiconductors and electron-injecting metals.
Consequently, the diodes require protection by means of an impermeable layer forming a barrier to oxygen and water vapor, which impermeable layer envelops the layers of the diode, and a support for the enveloped diode, preferably of high transparency, and which is impermeable, providing a barrier to oxygen and water vapor.
Thus far glass plate has been the support of choice, since it has excellent barrier and transparency properties. On the other hand, glass plate has the drawbacks of brittleness, high-weight, and rigidity.
A strong demand exists for plastic-film, both as the impermeable protective layer and as support material for the devices, since these may bring flexibility, high impact resistance, low weight, and, most of all, may enable roll-to-roll processing, as opposed to batch processing which has been used thus far. Such plastic film should, of course, be essentially impermeable, displaying low oxygen and water vapor transmission rates.
Although one may expect some further improvement in oxygen and moisture resistance of organic semiconductors employed in the diodes, extremely water-sensitive electron-injecting metals such as Ca, Li and Mg seem to be irreplaceable until a major breakthrough is made in solid state physics or in display design, both rather unlikely in the predictable future.
Other properties, which the envelope materials for organic displays should present, such as thermal resistance, low roughness, and low costs, are listed in [J. K. Mahon et al., Society of Vacuum Coaters, Proceedings of the 42nd Annual Technical Conference, Boston 1999, p. 496]. Organic photovoltaic devices also require similar, flexible, barrier materials, so do liquid crystal flexible displays, where barrier requirements are, however, less demanding.
Organic displays are proposed for such equipment as high-resolution computer displays, television screens, cell-phones and advanced telecommunication devices, etc., which require μm-scale precision manufacturing, vacuum operations and lithography. In other words: technologies similar to those at present used in microelectronics. Other applications include large scale displays for advertising and entertainment, and various communication devices. These latter applications may require lower precision in manufacturing, processing in inert-dry atmospheres, roll-to-roll operations, inexpensive methods of patterning, for example, stamping or ink jet printing. In other words: low-cost technologies, perhaps similar to those at present used in special quality graphic-printing.
The problem is thus to develop flexible polymer films which are essentially barriers to oxygen and water vapor and which can be produced at low thickness sufficient for their envelope functions, and such that they can be readily employed in commercial manufacture of the organic devices, preferably in roll-to-roll processing.
In order to satisfy market requirements, a polymer film for an OLED would need to limit permeation of oxygen and water molecules to such extent that the lifetime of a typical device is at least 10,000 hours.
It is known in the flexible packaging art, to coat polymer films or sheets with thin inorganic coatings, for example, metal oxide coatings, to render the polymer films or sheets essentially impermeable to oxygen and water vapor. In practice it is impossible in commercial manufacture to produce such coatings without some pinholes or other defects which permit passage of oxygen and water molecules through the otherwise impermeable coating. This may not be a serious problem in the flexible packaging art where the packaging is typically protecting a food product of limited shelf life. However, the levels of permeability that may be acceptable in the short working life of flexible packaging in the food and other industries will certainly not meet the more exacting requirements for organic displays based on organic light emitting diodes, which must have a life of years rather than the days or weeks which represent the typical useful or working life of flexible packaging.
A typical, transparent barrier-coated material consists of a substrate, usually made of plastic, and a single, very thin layer of a barrier material usually made of metal oxide, a mixture (an alloy, or a compound) of at least two metal oxides, or amorphous or polycrystalline carbon. Usually, the barrier material, for example metal oxide, is very hard, for example 2–10 GPa, as described in U.S. Pat. No. 6,083,313. This, however, has a drawback of inevitable stresses in the coating, usually compressive stresses, intrinsic to the method of its deposition, for example Physical Vapor Deposition (PVD). U.S. Pat. No. 5,718,967 describes a coating consisting of organic and inorganic layers, where the first layer being in direct contact with the substrate is an organic layer, more especially an organosilicon. This organosilicon layer essentially functions as a primer, and is deposited in absence of oxygen and provides adhesion improvement between the substrate and an inorganic (SiOx) coating.
U.S. Pat. No. 6,146,225 describes a multilayer structure consisting of organic coating, typically acrylic, deposited using PML technology (U.S. Pat. Nos. 5,260,095; 4,954,371; 4,842,893; 6,413,645) and an inorganic layer, typically an oxide or nitride, deposited using ECR-PECVD method.
U.S. Pat. No. 5,725,909 describes depositing a multilayer structure composed of an acrylic primer and a barrier-providing material such as SiO2, Al2O3 or a metal. U.S. Pat. No. 6,203,898 proposes depositing multilayer coatings, by plasma polymerization of condensable organic material, e.g., mineral oil or silicone oil, where a first layer comprises a carbon-rich material, and a second layer comprising silicon has no C—H or C—H2 IR absorption peaks.
In all above US patents, the inorganic coatings are not in direct contact with the substrate material such as plastic film, but with an organic, organosilicone or organic containing layer that has been deposited thereon.
U.S. Pat. No. 4,702,963; and EP 062334 describe flexible polymer film having inorganic thin film deposited thereon, where the multilayer coating is composed of inorganic materials. The first inorganic material, adhesion improving layer, comprises a coating of an elemental metal, for example Cr, Ta, Ni, Mo or SiO2 with >20% Cr; and the second, barrier-material, comprises a coating of a metal oxide, for example SiO, SiO2, MgO, CaO or BaO. These coatings, however, are not entirely transparent, as required, for display applications, such as in an OLED.
The present invention describes multilayer coatings that are distinct from multilayer coatings described in the literature. According to the present invention, the inorganic barrier-providing coating is in direct contact with the substrate. Multilayer coatings presented in the literature have acrylic, organosilicon or organic layers deposited from mineral or silicone oil, organic precursors, etc., usually as smoothing, stress-release or adhesion-improvement layers. Those coatings, however, typically show a relatively high permeability to gases and vapors.
Plasma coatings, deposited from organic- and organosilicon precursors, typically exhibit high permeation of gases and vapors. In special conditions, where carbon or silicon carbide layers are deposited, the coatings show low transparency to visible light and to near-IR and near-UV radiation.
U.S. Pat. No. 6,083,313 describes coatings of essentially high hardness, 2–10 GPa, whereas barrier materials according to present invention typically have a hardness of less than2 GPa. Other patents, such as U.S. Pat. No. 4,702,963 describe the coatings that have insufficient transparency for several important applications of the present invention, such as barrier coatings for flat panel displays, photovoltaic devices, and organic light-emitting sources.